adds ssl client cert key password

[raytung]

Hey team and happy new year!

This PR fixes https://github.com/fluent/fluent-plugin-kafka/issues/459 and also properly marks ssl_client_cert_key_password as secret in out_rdkafka2 as I just happen to stumble upon it.

[raytung]

@ashie I've rebased master thanks

[ashie]

Thanks! I'll create a new pull request to fix indentation & etc around this code.

[awulms-swift]

Thanks